The runtime is where all your content is stored.  The Poser library.

People often find Poser slowing down after awhile because they keep adding new stuff to their runtime - their library.  The bigger the library, the slower Poser runs.

Poser 5 and above has the ability to create and use external libraries - stored anywhere you like.  You may find doing this, or finding some other way to reduce the size of your runtime folder, will speed things up.